Do my certified documents need an apostille for Liechtenstein?

Usually yes, if your country is also a party to the Hague Apostille Convention, as Liechtenstein is. A notary certifies your signature or copy at home, and the competent authority there adds the apostille. If your country is not a party, the documents need consular legalisation instead, which takes longer.

Certification and apostille

Source: Status table: Convention of 5 October 1961 abolishing the requirement of legalisation (Apostille Convention), Hague Conference on Private International Law (HCCH)